The bones of the tarsus or ankle (fig. 16). are seven in number, and are arranged in three transverse rows—a proximal, consist ing of the astragalus and os calcis, a middle, of the scaphoid and a distal, consisting of the cuboid, ecto-, meso- and ento-cuneiform. The tarsal, like the carpal bones, are short and, with the exception of the cuneiforms which are wedge-shaped, irregularly cuboidal; the dorsal and plantar surfaces are as a rule rough for ligaments, but as the astragalus is locked in between the bones of the leg and the os calcis, its dorsal and plantar surfaces, as well as the dorsum of the os calcis, are smooth for articulation ; similarly, its lateral surfaces are smooth for articulation with the two malleoli. The posterior surface of the os calcis projects backward to form the prominence of the heel. With this exception, the bones have their anterior and posterior surfaces smooth for articulation. Their lateral surfaces are also articular, except the outer sur face of the os calcis and cuboid, which form the outer border ; and the inner surface of the os calcis, scaphoid and the ento-cunei form, which form the inner border of the tarsus. Supernumerary bones are occasionally found as in the hand.
The metatarsal bones and the phalanges of the toes agree in number and general form with the metacarpal bones and the pha langes in the hand. The bones of the great toe or hallux are more massive than those of the other digits, and this digit, unlike the thumb or pollex, does not diverge from the other digits, but lies almost parallel to them.
Embryology.—The development of the appendicular skeleton takes place in the core of mesenchyme in the centre of each limb. By mesenchyme is meant that part of the mesoderm, or middle layer of the embryo, in which the cells are irregularly scattered in a matrix, and are not arranged in definite rows or sheets as in the coelomic membrane. This substance first becomes changed into cartilage, except perhaps in the case of the clavicle.
The factors which determine the general shape and proportion ate size of each limb bone are at work while the cartilage is being formed, because each future bone has a good cartilaginous model laid down before ossification begins. Calcification usually begins at one point in each bone, unless that bone be a compound one formed by the fusion of two or more elements which were distinct in lower vertebrate types, as is the case with the os innominatum.
Calcification, once established, acts as an attraction for blood vessels, which probably bring with them osteoblasts, and the subsequent ossification is a process which needs and receives a plenteous supply of nourishment. After a long bone has reached
a certain size it very often has extra centres of ossification (epiphyses) developed at its ends as well as at places where impor tant muscles have raised lever like knobs of cartilage on the model.
Turning now to the develop ment of the individual bones of the axial skeleton, the clavicle is partly fibrous, and partly carti laginous; its primary centre is the earliest of all in the body to ap pear, while its sternal epiphysis does not come till the bone is fully grown, and so can have no effect on the growth of the bone. It is probably atavistic, and is often regarded as the vestige of the precoracoid, though it may represent the inter-clavicle. It sometimes fails to appear at all.
The centres for the scapula are shown in the accompanying fig ures (fig. 19). G. B. Howes re garded the subcoracoid centre as the atavistic epiphysis represent ing the coracoid bone of lower vertebrates, while the human coracoid he looked upon as the equivalent of the epicoracoid. The epiphyses in the vertebral border are atavistic and represent the supra-scapular element (see section on Comparative Anatomy).
In the humerus the centre for the shaft appears about the eighth week of foetal life, which is the usual time for primary centres.
The ulna is a very interesting bone. The upper epiphysis shown in fig. 21 does not encroach upon the articular surface, but is developed in the triceps tendon and is serially homologous with the patella in the lower limb.
In the radius there are two terminal epiphyses and one for the insertion of the biceps.
The carpus ossifies after birth, one centre for each bone occur ring in the following order : os magnum, 1 i to 12 months ; unci form, 12 to 14 months; cuneiform, 3 years; semilunar, 5 to 6 years; trapezium, 6 years; scaphoid, 6 years; trapezoid, 6 to 7 years; pisiform, lc) to 12 years.
The metacarpal bones have one centre each for the shaft and one epiphysis for the head, except that for the thumb which has one centre for the shaft and one epiphysis for the proximal end.
The phalanges develop in the same way that the metacarpal bone of the thumb does.
The os innominatum has three primary centres for the ilium, ischium and pubis.
The special centres for the crest of the ilium are probably a serial repetition of those for the vertebral border of the scapula (see fig. 19). The centre for the pubic symphysis probably repre sents the epipubis of amphibians, while that for the tuberosity of the tendon of which it is formed, is defined. Its ossification begins in the third year. The patella is usually looked upon as the largest and most typical example of a sesamoid bone in the body.